This invention relates to adducts of carboxylic acids and isocyanates with epoxides, to aqueous dispersions containing such adducts, to a process for the preparation of such epoxy resin dispersions and to their use in the autophoretic coating of metallic surfaces.
Metallic surfaces, particularly the surfaces of metal components used in the automotive industry, are coated with a first base lacquer for protection against corrosion and to improve the adhesion of decorative surface lacquers. This base coat is applied to the metal surface by a process known as "cathodic electrodeposition" (CED). It consists of polymers bearing cationic groups which are generally based on an epoxy resin and contain a hardener which is intended to bring about crosslinking of the polymer molecules and, hence, a sealing of the lacquer layer through hardening. Base coats applied by CED provide the metal surfaces with good protection against corrosion. For this reason alone, CED is used worldwide for applying base coats to metal surfaces.
However, despite good results in terms of corrosion prevention, cathodic electrodeposition is attended by serious disadvantages. Thus, any voids in the metal workpiece can only be very inadequately coated. In addition, the process requires the application of an electrical voltage for deposition of the resin to the metallic surface. As a result, plants for the CED process are extremely expensive and, in addition, require elaborate safety measures for personnel working on the plant.
2. Statement of Related Art:
The disadvantages mentioned above do not attend processes for the autophoretic coating of metallic surfaces. This process uses emulsion polymers, for example based on acrylate or styrene/butadiene, which are anionically stabilized. The particle sizes are typically in the range from 80 to 150 nm. The dispersions show adequate acid stability for the process. The organic polymers are normally deposited on the metal surface using a so-called "starter" of HF/FeF3. Autophoretic chemical coating (ACC) processes such as these are described in a number of publications, of which DE-PS No. 26 12 995 and U.S. Pat. Nos. 3,585,084, 3,839,097, 4,318,944 and 4,347,172 are mentioned by way of example. However, compared with CED processes, ACC processes are still attended by the disadvantage that the polymer cannot be crosslinked on the metal surface through the incorporation of hardeners in the polymerization mixture. This results in layers which are permeated by "pores" and which thus make the metal beneath the layers more susceptible to corrosion than layers applied by CED. Although bridging or rather crosslinking complexing of the polymer molecules can be obtained by subsequently rinsing the layers applied by the ACC process with an aqueous bath containing chromium ions, it is desirable on ecological and toxicological grounds to limit or even completely avoid the use of chromium-containing rinsing solutions.
By contrast, one of the objects of the present invention is to provide polymers suitable for the ACC process which form crosslinked layers that afford effective protection against corrosion, even without the use of chromium-containing rinsing solutions, and in addition, are solvent-resistant.
Polymers intended for use in autophoretic coating have to be very finely dispersible in water (particle size below about 300 mm). In addition, dispersion and polymer must be stable in the pH range from 1.5 to 8. Dispersions based on epoxy resins and hardeners (crosslinking agents) which satisfy these requirements have not hitherto been available. The epoxy resins used in cathodic electrodeposition cannot be used in autophoretic coating processes.
Accordingly, another object of the present invention is to provide finely divided hardener-containing epoxy resin dispersions which are stable at pH values in the range from 1.5 to 8 and which can be applied by autophoretic coating, giving crosslinked, corrosion-resistant and solvent-resistant coatings without the additional use of chromium-containing rinsing solutions.